Heat pipe furnace

ABSTRACT

A furnace having a constant uniform temperature zone is provided by a heat pipe so shaped that the outside walls of the heat pipe form the inside walls of the furnace. A particular embodiment for growing crystals provides one or more annular shaped heat pipes heated at one end with the cylindrical space enclosed by the annular heat pipe at constant uniform temperature for heating crystal growing material, with access to the crystal growing material through the ends of the cylindrical space.

United States Patent [191 Steininger et al.

[451 Dec. 31, 1974 HEAT PIPE FURNACE Inventors: Jacques Steininger,Lexington;

Thomas B. Reed, Concord, both of Mass.

Massachusetts Institute of Technology, Cambridge, Mass.

Filed: June 27, 1973 Appl. No.: 374,223

Related US. Application Data Division of Ser. No. 241,597, April 6,1972.

Assignee:

US. Cl 13/22, 13/1, 219/397 Int. Cl H05b 3/66, F28d 15/00 Field ofSearch 13/1, 24, 22,20; 219/399, 219/395, 406, 397, 530, 540, 390, 413;165/105 References Cited UNITED STATES PATENTS 3/1972 Kirkpatrick 13/22Primary Examiner-R. N. Envall, Jr. Attorney, Agent, or Firm-Arthur A.Smith, Jr.; Martin M. Santa; Robert Shaw ABSTRACT A furnace having aconstant uniform temperature zone is provided by a heat pipe so shapedthat the outside walls of the heat pipe form the inside walls of the vfurnace. A particular embodiment for growing crystals provides one ormore annular shaped heat pipes heated at one end with the cylindricalspace enclosed by the annular heat pipe at constant uniform temperaturefor heating crystal growing material, with access to the crystal growingmaterial through the ends of the cylindrical space.

6 Claims, 11 Drawing Figures CRYSTAL "56 FULLER DRlVE RF POWER souncz uan PATENTEU macs 1 I574 SHEET 20F 4 CRYSTAL PULLER DRIVE NE ALONG AXIS50 FROM P TO BOTTOM m R 7 EE 4 WC 8 P QR M PW T m R E m H. D 5 5 RAW -HA a 7 FIG '6 PATENTEU 951331 3 4 TEMPERATURE ALONG FURNACE AXIS (C)3.857. 990 SHEET 3 BF 4 FURNACE WITH l5-cm QUARTZ-ZINC ANNULAR HEAT PIPE0 HIGH POWER x LOW POWER X l I l 0 5 I0 l5 DISTANCE FROM BOTTOM OF HEATPIPE cm) PATENTEDUEBIN I914 35857. 990

saw u or TEMP DISTANCE ALONG AXIS 70 FROM TOP TO BOTTOM ssfw w H OOOOQOOOOOO DISTANCE ALONG AXIS 80 FROM TOP TO BOTTOM HEAT PIPE FURNACEfiled Apr. 6, 1972.

This invention relates to furnaces, and more particularly to a source ofuniform constant temperature'forming the inside walls of a furnace.Heretofore, heat pipes have been used as highly efficient heatexchangers and have been used where the high thermal conductance of theheat pipe can be exploited. The heat pipe is essentially a closedvessel, lined internally witha wick saturated with a volatile liquid.Heat is absorbed by evaporation of the liquid in high temperatureregions of the pipe and transferred by vapor transport to colderregions, where it is released by condensation of the vapor. Thecondensed liquid is then recycled by capillary action in the wick andflows back to the point of high temperature where it is againevaporated. Because of the high latent heats of vaporization of liquids,effective thermal conductivities several orders of magnitude greaterthan those of the best conducting metals can be obtained and it is thischaracteristic which has spurred the principal interest in heat pipes inthe past.

Heretofore, the operating temperatures of heat pipes been a metal, suchas lithium, zinc, or sodium. How- I ever, the principle of operation toprovide high thermal conductance can be extended to much lowertemperatures employing other liquids to provide the high conductanceat-ambient temperatures or lower.

In the flow cycle inside a heat pipe, the evaporation and condensationconstitutes a mono-variant reversible phase transformation and so theheat pipe tends to assume a very nearly isothermal profile from one'endto the other. It is this characteristic of a heat pipe which is animportant part ofthe present invention. It is an object of thisinvention'to provide afurnace having at least one zone of constantuniform temperature with access to the zone so that crystals can begrown therein, for example, by pulling the crystals from a melt.

It is another object of the present invention to provide a furnace linerfor maintaining a zone of constant uniform temperature inside thefurnace.

It is another object to provide a surface of substantial area ofconstant uniform temperature for heating bodies in contact therewith.

It is another object to provide means for enclosing an object andmaintaining the temperature thereof constant.

It is another object to provide a hot plate for use in open countercooking.

It is another object to provide a furnace having at least two adjacentzones, each of different constant uniform temperature.

It is a further object to provide such a multiple zone furnace withaccess to the zones through the ends of the furnace to permit movingcrystal growing material between the zones and drawing crystals from amelt of the material.

In accordance with the principalfeature of the present invention, afurnace liner is provided enclosing a zone of constant uniformtemperature in the furnace, the liner being made at least partially of aheat pipe, the heat pipe being so shaped that the outside walls of theheat pipe form at least a part of the walls of the zone of constant,uniform temperature inside the furnace. In

particular embodiments of the invention, a complete furnace liner isprovided by a heat pipe consisting of two concentric cylinders, oneinside the other, with the space between the cylinders forming theinside of the heat pipe and the inside of the inner cylinder definingthe zone of constant uniform temperature in the furnace.

These and other objects and features of the present invention arerevealed by the following specific description of embodiments of theinvention, taken in' to provide a hot plate, used for example forcounter top cooking;

FIG. 5 is a partially sectioned view of a multi-zone furnace for heatingcrystal growing'material and growing crystals therefrom;

FIG. 6 is a plot of temperature along the furnace axis v distance alongthe axis for an embodiment of the present invention, illustrating theuniformity of temperature; I

FIG. 7 is a chart of temperature v distance along the axis of theapparatus in FIG. 5;

FIG. 8 is a partially sectioned view of apparatus with three zones forzone melting processes;

FIG. 9 is a chart of temperature v distance along the I outsidecylinders 2 and 3, which are closed at their ends. The annular space 4between the cylinders is lined or filled with wicks made up of severallayers of fine mesh screen 5. A fluid is also contained in the annularspace and selected in consideration of the temperature desired.

The liner 1 is enclosed by furnace insulation 6 in a container 7 and anaxial opening 8 at one end, concentric with the axis 9 of the liner isprovided for access to the constant temperature zone 10 inside thefurnace, defined by the inside walls of the cylinder 2. A plug'll may beprovided for closing the opening 8.

Heat is delivered to the liner by a heating coil 12, wound around oneend of the liner and energized by a power source 13. Heat is carriedfrom the coil to the liner for evaporating the liquid inside the annularspace 4 either by radiation, conduction or a combination of both.

In operation, the heat supplied causes evaporation of the liquid in thespace 4 in the high temperature region 14 of the space. Due to thepressure differential inside the annular space 4 this vapor istransported at sonic velocity in the direction of arrow I5 toward thecolder regions of the pipe at the end adjacent the opening 8 andcondenses at the colder end or along the way toward the colder end,releasing absorbed heat of vaporization. The condensed liquid is thenrecycled to the high temperature evaporating area 14 by capillary action.in the wick. This may be assisted by gravity if the furnace stands withthe axis 9 vertical and the high temperature region 14 at the bottom.

The actual rate of heat transfer in the annular space 4 is essentiallyequal to the latent heat flux in the evaporator, expressed as follows:

where:

Q is heat transfer rate,

rh, is vapor mass flow rate L is latent heat of vaporization of theliquid.

For an annular channel, such as channel 4 in the structure-in FIG. 1,equation 1 becomes:

Q PP

where:

A, is the cross section area of the annular space, p is the vapordensity, and V is the average vapor velocity Because of high latent heatof vaporization of the selected liquid, effective thermal conductivitiesseveral mum heat transfer rate limited by the maximum flowrate of thecondensed liquid through the wick can be expressed for the liner shownin FIG. I by:

where:

ais surface tension of the liquid A is the free flow area of the wick Dis the wick pore size 1/ is kinematic viscosity, and

b is capillary geometric constant (about 20 for wire mesh capillary) Thefurnace liner, such as shown in FIG. 1, can be made and operated ofselected materials to provide a constant temperature zone ranging frombelow C to above 2,000C, depending on the working liquid that isselected. Working liquids can have vapor pressures at their operatingtemperatures ranging from a few Torrs to several atmospheres. Theselection of the container, the wick materials and the liquid is subjectto the requirement that the liquid must wet but not react with the othermaterials. The table below shows combinations of materials and liquidsthat are suitable for operation at various temperature ranges indicated.

Table l-Continued Fluid Material Temperature Hydrocarbons MercuryStainless Steel 350 Potassium Nickel 600 Sodium Nickel 900 StainlessSteel 780 Hastelloy 750 Zinc Quartz llOO Lithium TZM Alloy I500 Tungstenl500 The furnace liner illustrated in FIGS. I and 2 for providing thezone of constant uniform temperature in a furnace is generallycylindrical and the zone is cylindrical. This is a convenient shape andpermits relatively easy construction of the furnace liner and thefurnace from stock materials. However, the liner may be square orrectangular in cross section to provide a square/rectangular shaped zoneinside the furnace, such as illustrated by the liner 21 in FIG. 3,enclosed by insulation 22 and heated at one end by a heating coil 23,energized by source 24. Clearly, the liner can be just about any shape,or a portion of an inside wallofa furnace can be a section of heat pipe,taking advantage of the constant uniform temperature profile across thesurface of the heat pipe and the intrinsic temperature reliabilityassociated with the temperature of evaporation and condensation of apure liquid.

FIG. 4 illustrates use of these same qualities to provide a hot platesurface 26, this being one face of a flat heat pipe 27, supported by athermally insulating base 28, and heated at one end by, for example, aheating coil 29, energized by a power source 30. Inside the flat heatpipe '27, a wick 31 extends throughout the inside and carries condensedliquid back to the hot area adjacent the heating coil 29, where liquidis evaporated absorbing heat and carrying the heat at very highefficiency throughout the surface 26 of the plate. The hollow inside ofthe plate (or cavity) containing the wick 31 and fluid is preferablysupported throughout by structure such as 32 to maintain the mechanicalshape even while pressure inside is varied in control of temperature.This embodiment is suggested for use, for example for counter topcooking, as it provides a surface 26 of constant uniform temperature,determined substantially by the selected liquid. Here, high thermalconductivity of the heat pipe is also exploited, as this characteristicinsures that heat from the coil is efficiently carried to all points ofthe surface 26. It is not dependent upon the heat carrying capacity ofthe liner material per se.

From the thermodynamic point of view, evaporation and condensation of apure liquid constitute-a monovariant reversible phase transformation.Heat pipes tend to assume nearly isothermal temperature profiles and inpractice a small pressure drop is needed" to maintain the flow of vaporfrom the evaporator (hot end) of the pipe to the condenser (colder end).This pressure drop, however, can be as small as l torr and so quiteobviously the temperature gradient from one end to the other isextremely small. It should be noted, however, that the pipes can beoperated in two modes: constant volume and constant temperature. If thepipe is sealed as the embodiments described herein, operation is atconstant volume and variations in input power are matched by variationsin internal vapor pressure and temperature. Thus, the closed or constantvolume, furnace liner or hot plate, such as described above, can beoperated over a temperature range dependent upon the input power andthis is accompanied by an increase in pressure inside the pipe. If inputpower is monitored by pressure inside the pipe, the power can becontrolled by a feedback arrangement to maintain temperature constant.For example, in the hot plate embodiment shown in FIG. 4, pressureinside the pipe could be sensed by a pressure sensing mechanism,producing an output for controlling the electrical energy from the powersource 30. This would constitute a feedback circuit and could beselectively set, thereby setting the temperature of the plate 26 over apredetermined temperature range. The temperature range would bedetermined in consideration of the rupture pressure of the flat heatpipe 27.

It is also possible to operate a heat pipe as an open pipe where thevapor is contained by an inert gas pressure. In that case, variations inpower input are matched by variations in the length of the vapor zone inthe heat pipe and the pipe operates with an isothermal zone offixedtemperature, but of variable length. This fixed temperature issubstantially independent of the amount of input power, at least over asignificant range.

Use of the annular heat pipe, such as the liner 1 in FIG. 1 to provide amulti-zone temperature profile along the axis of a heating zone isillustrated by the structure in FIG. 5. This is particularly designedfor crystal growing or annealing. With several annular heat pipes inseries, well defined short zones with extremely high temperaturegradients in between 'can be obtained along a crystal growing axisfTheapparatus in FIG. 5 provides such temperature zones for producing a meltof crystal growing material in one zone and drawing crystals from themelt into a lower temperature zone, at a temperature maintained toprevent constitutional supercooling and to minimize thermal strain inthe drawn crystal. In this apparatus,.the upper zone 41 is maintained byheat pipe liner 42 and the lower zone 42 is maintained by liner 44. Theliners are separated by a thin spacer 45 and heated by an RF coil 46,energized by RF power source 47. Along the axis 50 of the liners islocated a pedestal 51 for supporting the melt 52 in zone 43. A crystal53 is pulled from the melt into zone 41, where the temperature ismaintained ideal for cooling. The apparatus for pulling the crystal mayinclude a seed rod 54, with an extension 55, to a crystal puller drive56.

The heat pipe liners 42 and 44 are held rigidly together and supportedby frame 57, which is positioned along the axis 50 by liner drive 58.

FIG. 6 is an approximate chart of temperature along the axis 50, showingthe zones 41 and 43 and the transition in between. The constanttemperature zones denoted T and T are shown flat in the chart, althoughthey may have a slight gradient on the order of a degree of two percentimeter. However, the transition zone at feet position the pulledcrystal 53 for ideal growth as the crystal is pulled from the melt 52.

A typical heat pipe liner, such as 42 or 44, shown in FIG. 5, designedfor operating in the temperaturerange of l,l00C may include twoconcentric cylinder sections 61 and 62, which are of quartz. Quartz isparticularly useful because it is workable and transparent, permittingobservation of the inside of the liner. The wicks 63 in this liner areofquartz cloth and the working liquid can be either zinc or cadmium. Afeed tube such as 64 at the end of the liner serves to load the liquidand afterwards is sealed off to provide the constant volume heat pipeliner.

Accurate measurements of the temperature profile along the axis of sucha quartz heat pipe liner in a furnace at different input power levelsare plotted in FIG. 6. These plots were obtained with a double walledquartz pipe 3.4 centimeters outside diameter, 1.9 centimeters insidediameter and 15 centimeters long. The wicks are of quartz woven clothand the working liquid is zinc. Two temperature profiles measured alongthe axis at high and low input powers are plotted. As can be seen fromthe plot, the higher temperature profile remains within a few degrees ofl,065C for a distance of 8 centimeters, where it reaches a cold zone atthe upper part of the liner, which is attributed to the presence ofnon-condensable residual gases inside the liner. These gases apparentlyare swept to the colder section by the pumping action of the zinc vaporand form a low conductivity vapor lock.

Other combinations of materials and fluids for designated temperatureoperation are listed in the Table 1 above. A liner constructed of nickelis particularly sturdy, containing several layers of mesh nickel wirescreen to provide the wick. For a temperature of 900C, the working fluidmay be high purity sodium, which is loaded into the liner by vacuumdistillation through the feed tubewhich is then pinched and electronbeam welded.

FIGS. 8 and I0 illustrate in less detail other apparatus for crystalgrowth. In FIG. 8, three heat pipe liners 71,

72 and 73 abut end-to-end, separated by spacers 74 and 75, and areheated by IF coil 76 enclosing portions of each and energized by RFsource 77. Along the axis 70, the liner 72 provides a short,high-temperature zone, denoted T between two lower temperature zones Tand T This is shown in the chart in FIG. 9. A boule of crystal growingmaterial 78 on a pedestal 79 is raised by drive 80 into the center zoneT and so zone melting occurs in the crystal, producing uponcrystallization the desired crystalline structure along that zone.

The apparatus in FIG. 10 includes two'heat pipe liners 81 and 82 in anRF heated furnace 83, the furnace walls being lined with an RF heatingcoil 84, energized by RF source 85. Three zones are produced in thisfurnace, the upper zone 86, the zone inside liner 81 and the zone insideliner 82. A body 87 such as a crystal lowered along the axis of theapparatus by a drive 88 will experience first the lowest temperature Tthen the highest temperature T and then an intermediate temperature T asshown on the chart in FIG. 11. This sequence of temperatures is desiredwhere crystal growing vapor material is introduced inside the linerswhile the crystal 87 moves along the axis 80, as in a vapor phasecrystal growth process.

The uses of the heat pipe liner in apparatus described herein, areexamples of but a few particular uses related to crystal growth. Theseuses have been foremost, because the heat pipe liner provides a uniform,very even temperature zone, which can be precisely controlled and is sonecessary in the growth of pure crystalline materials. Clearly, otherheat pipe configurations besides the liner configuration for providing azone of known dimensions in which temperature is precisely controlledand extremely uniform have application in other areas. For example,small, but uniform temperature gradients over relatively large areas forepitaxial semiconductor growth could be accomplished either with an offaxis annular heat pipe or a sandwich configuration of two flat heatpipes maintained at slightly different temperatures. These and otherapplications of the present invention are within the scope of theinvention as set forth in the appended claims, in which:

We claim:

1. in a furnace, for providing at least two zones of different constantuniform temperature and a high temperature gradient therebetween,comprising,

a first and second hollow-walled sealed container defining a first andsecond zone in their interiors,

a wick inside the hollow wall extending from one end thereof to theother end thereof,

a liquid inside the hollow wall impregnating the wick,

said first and second containers being in axial alignment and having anend of each in proximity to one another,

an insulating spacer between said first and second container ends,

means for heating the liquid in both zones with a heater locatedadjacent the proximate ends of both hollow walls,

the liquids in the first and second hollow walls are different, havingdifferent temperatures of vaporization selected to provide differentzone temperatures,

the liquids absorb heat from the heater, evaporate, and fill the insideof the hollow walls with vapor at said constant different temperaturesand the vapor condenses, gives up heat to the wall, flows through thewick and is heated again to repeat the cycle,

whereby a high thermal gradient occurs across the insulating spacerbetween the first and second zones.

2. In a furnace as in claim 1 for use in growing crystalline materialscomprising in addition means for introducing crystal growing materialsinto the interior of one container,

means for moving the crystal growing material along the axis withrespect to the heat pipes,

means for pulling a crystal from the introduced material,

said containers including wick and liquid being designated as a heatpipe,

whereby said different temperature zones and the high thermal gradientare effective in the control of crystal growth from the materials.

3. In a furnace as in claim 2 wherein said containers are formed of twoconcentric cylinders with the annular space therebetween being closed atboth ends of the cylinders,

and the axis of the cylinders being the axis of the containers.

4. A furnace as in claim 1 wherein,

the heater is an RF radiation coil enclosing adjacent ends of the hollowwalls concentric therewith.

5. A furnace as in claim 2 wherein the means for heating is an RF coilaround at least a portion of each of the heat pipes attheir proximateends.

6. A furnace as in claim 5 wherein the heat pipes and RF coil aremovable with respect v UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3,857,990 Dated December 31, 1974 Inventor(s)Jacques Steininger and Thomas B. Reed It is certified that error appearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, after the first sentence, insert the following statement:

The invention herein described was made in the performance of work undera, contract from the United States Air Force, Electronic SystemsDivision.-

Signed and Scaled this eighteenth D ay Of November 1 9 75 [SEAL]Arrest.-

RUTH C. MASON C. MARSHALL DANN :I HSII'HZ ffl'fi (mnmixsimzer uj'Palenrsand Trademarks I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 3,857,990 Dated December 31, 1974 Inventor(s) JacquesSteininger and Thomas B. Reed It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, after the first sentence, insert the following statement:

-The invention herein described was made in the performance of workunder a contract from the United States Air Force, Electronic SystemsDivision.

Signed and Scaled this eighteenth D ay 0f November 1 9 75 [SEAL] Arrest.

RUTH C. MASON C. MARSHALL DANN .lmsring ()jfl'cer (mnmissivner uflarenrsand Tradcmurkx

1. In a furnace, for providing at least two zones of different constantuniform temperature and a high temperature gradient therebetween,comprising, a first and second hollow-walled sealed container defining afirst and second zone in their interiors, a wick inside the hollow wallextending from one end thereof to the other end thereof, a liquid insidethe hollow wall impregnating the wick, said first and second containersbeing in axial alignment and having an end of each in proximity to oneanother, an insulating spacer between said first and second containerends, means for heating the liquid in both zones with a heater locatedadjacent the proximate ends of both hollow walls, the liquids in thefirst and second hollow walls are different, having differenttemperatures of vaporization selected to provide different zonetemperatures, the liquids absorb heat from the heater, evaporate, andfill the inside of the hollow walls with vapor at said constantdifferent temperatures and the vapor condenses, gives up heat to thewall, flows through the wick and is heated again to repeat the cycle,whereby a high thermal gradient occurs across the insulating spacerbetween the first and second zones.
 2. In a furnace as in claim 1 foruse in growing crystalline materials comprising in addition means forintroducing crystal growing materials into the interior of onecontainer, means for moving the crystal growing material along the axiswith respect to the heat pipes, means for pulling a crystal from theintroduced material, said containers including wick and liquid beingdesignated as a heat pipe, whereby said different temperature zones andthe high thermal gradient are effective in the control of crystal growthfrom the materials.
 3. In a furnace as in claim 2 wherein saidcontainers are formed of two concentric cylinders with the annular spacetherebetween being closed at both ends of the cylinders, and the axis ofthe cylinders being the axis of the containers.
 4. A furnace as in claim1 wherein, the heater is an RF radiation coil enclosing adjacent ends ofthe hollow walls concentric therewith.
 5. A furnace as in claim 2wherein the means for heating is an RF coil around at least a portion ofeach of the heat pipeS at their proximate ends.
 6. A furnace as in claim5 wherein the heat pipes and RF coil are movable with respect to eachother along the axis.